Product Viscosity and the Impact on Liquid Filling Machines

A simple definition of viscosity refers to the resistance to flow of a particular liquid. Fluids with low viscosity may be deemed "free-flowing", while those with high viscosity can be referred to as "thick". Measured in centipoise (cp), water, for example, has a viscosity of 1 to 5 cps, while honey ranges from 2000 to 3000 cps. In the world of packaging, the viscosity of the liquids being prepared will influence the choice of filling machine for any given project.

While other factors will be considered when searching for a filling solution, product viscosity is usually the best place to start, given that certain types of filling machines are better suited for different viscosities. Gravity fillers and overflow filling machines tend to be built for low viscosity, or free-flowing products, though for different reasons. Gravity fillers use a raised tank and a time-based fill relying on gravity alone to move liquid from a high position into waiting bottles. Overflow filling machines recirculate liquid back to a holding tank once a pre-set level is reached in each bottle. In both cases, higher viscosity liquids would normally result in an inefficient filling process.

For projects with thicker liquids, pump and piston fillers offer an extra push. Pump fillers match the pump type to the product to help move higher viscosity liquids through the product pathway. While piston fillers allow thick products to be pulled into an empty cylinder, then forced out of the cylinder by the reentry of the piston. For these projects, pumps and pistons provide support in moving liquids that do not flow freely to add efficiency to the filling process. While there are definitely exceptions to these general rules based on other factors, gravity and overflow fillers will typically run low viscosity products while piston and pump fillers will be manufactured for projects with higher viscosity liquids.

As shown above, the viscosity of a liquid will often include a range, such as honey ranging from 2000 to 3000 cps. This range stems from the fact that the flow of liquids can change as the temperature of the liquid changes. When using a liquid filling machine, even slight changes in viscosity can affect the filling process. When the flow of a product changes, the settings of the filling machine may also need to change to keep the process uniform and the machine performance reliable. The simplest solution for this issue is to perform the packaging process in a controlled atmosphere where the temperature can be controlled or at least kept within a reasonable range. When not possible, simple or subtle changes to the filling machine settings may be required when temperature rises or falls.

For any packaging project, factors such as bottles, production space, range of liquids, and demand will also help a business find the ideal filling machine for the project, but product viscosity will always play a part in the search for a consistent and efficient filling process.